At present, light emitting diodes (hereinafter abbreviated as LEDs) are applied for illumination purposes due to for example their high energy efficiency, ability to produce different colors by mixing of for example blue, red, green and white LEDs, dimmability, compact dimensions or for other reasons. This trend is supported by the increasing availability of high power LED's at relatively low cost. In order to drive the LED's, use may be made of a driver circuit, that may include a controller that controls an intensity of the or each LED by means of duty cycling, pulse width modulation, pulsation, LED current control or others or any combination thereof. Furthermore, a current source or similar circuit may be comprised in the drive circuit. In order to drive the LEDs at a variety of colors, intensity, etc, use may be made of a digital communication network, such as a DMX network, that communicates data to one or more drivers. Commonly, a sequence of data is provided via the network to the driver in order to provide information regarding for example intensity, color, etc to the driver circuit.
Current solutions for driving LEDs have difficulties in controlling the LED's current in such a way that the intensity of the radiated light changes in a smooth way as perceived by humans when the intensity set-point is changed. Causes are a.o. the network bandwidth through which set-points arrive at the driver circuit combined with a certain size of changes between sequential set-points but can also be the speed and step size of set-points as provided by directly connected human interfaces (such as a dimming wheel control). The same inability to achieve smoothness is often present when changing color.
A typical example is a LED driver circuit (the driver circuit may in this document also be referred to as driver) that is connected through a DMX network where new set-points for a LED may arrive at 23 ms distances in a network with 512 channels and 8 bit set-points. When the stepsize between 2 such neighbouring set-points is more than 1% of the light intensity, typical humans perceive this as a brightness discontinuity. On the other hand, certain steep brightness changes are desired. Achieving those steep changes may, using a current data network typically only be done using step sizes larger than 1% of the current brightness.
The result may be that humans may perceive discontinuities in the light's intensity or color.
To aggravate the situation, today higher demands are made with respect to resolution, contrast and update rate of the set-points as applications are broadening to “very low light”, “very dynamic”, “very low light as well as very bright light in 1 system” and video situations.
Also the number of channels used tends to increase.
These demands lead to higher stepsizes between 2 consecutive set-points, wider set-points (e.g. 32 or 16 in stead of 8 bits), higher set-point rates, multiple in stead of 1 frames to be sent to transfer the set-points for all channels, etcetera.
As an example, 16 bit set-points in a DMX network system would mean that the number of channels is either halved or, when insisting on 512 channels, that 2 frames are needed which increases inter set-point time to at least 46 milliseconds (not counting the interframe delay).
Another disadvantage of todays systems arises in video systems where synchronization issues arise when setpoints for pixels that belong to a video frame at time T are sent sequentially in time. The increasing delay per pixel causes visual effects in the picture finally shown to the user (for example an oblique line may appear to be a series of isolated dots on screen). Different delay sizes can occur depending on the network type. In DMX for example the distance between channel set-points within a frame differs from the extra delay that will occur between the last set-point in a current frame with respect to the first set-point in the successor frame.
Also most contemporary solutions support a LED current on/off rate that is in the range of 50 to 200 Hertz. For video applications however this is insufficient, as video frames may capture only light during OFF periods, or may have variations in the average light captured due to ON/OFF times that are to large percentage-wise in comparison to the video frame time. A LED duty cycle rate of more than about 300 Hertz may be required. The variations mentioned in this paragraph are noticeable in the video picture and are therefore a disadvantage of current systems.